396 



BOTANY 



PART II 



CONJUGATION, in the case of Spirogyra, is preceded by the development of 

 converging lateral processes from the cells of adjacent filaments. When two 



processes from opposite cells meet (Fig. 328^4) 

 their walls become absorbed at the point of 

 contact, and the whole protoplasmic contents 

 of one cell, after contracting from the cell wall, 

 passes through the canal which is thus formed 

 into the opposite cell. The protoplasm and 

 nuclei of the conjugating protoplasts then fuse 

 together while the chloroplasts do not unite, 

 but those of the entering protoplast disorganise. 

 The resulting cell forms the zygospore in- 

 vested with a thick wall, and filled with fatty 

 substances arid reddish-brown mucilage spheres. 

 This form of conjugation, which' is the one ex- 

 hibited by most species, is described as scalari- 

 form (Fig. 328 A), as distinct from the lateral 

 conjugation of some species, in which two 

 adjacent cells of the same filament conjugate 

 by the development of coalescing processes, 

 which are formed near their transverse wall 

 (Fig. 328 ). In some genera the zygote is 



formed midway in the conjugation tube. 



J . d 



The conjugation nucleus of the young zygo- 



spore undergoes a tetrad division associated 

 with the reduction in number of the chromo- 

 somes. One of the four nuclei becomes that of 

 the young plant while the others appear as 

 small nuclei, which then degenerate (Fig. 329). The chloroplasts of the gamete 

 that passed across also degenerate. In this way one young plant arises which 

 protrudes from the zygospore as a tubular growth and forms a filament by cell 

 division ( 26 ). 



FIG. 329. Spirogyra longata : zygotes of 

 various age. ' A, The two sexual nuclei 

 before fusion ; B, after fusion ; C, 

 division of the nucleus of the zygote 

 into four haploid nuclei ; D, the 

 three small nuclei degenerating. The 

 chloroplasts are represented as cut 

 across against the wall. (After 

 TRO'NDLE.) 



CLASS VIII 



Heteroeontae (*> n > 27 ) 



In the Heteroeontae a number of genera of green Algae are included which were 

 formerly placed in the Chlorophyceae but are now separated as an independent class 

 derived from the Chrysomonadinae. 



They are characterised by the yellowish green colour of the discoid chroma- 

 tophores, which contain in addition to chlorophyll a yellow pigment which turns 

 blue with acids, and form oil and not starch as the product of assimilation. The 

 motile cells almost always have two cilia of unequal length attached rather to the 

 side. The cell wall, which contains pectin and is usually silicified, in many cases 

 consists of two overlapping halves. Some Heteroeontae are unicellular, others 

 form gelatinous colonies, while others are filamentous. 



Reproduction is effected by swarm-spores, which in some genera are replaced 

 by endogenous aplanospores. Resting cysts are also formed, and these, like 

 the aplanospores, have a two-valved silicified membrane. In certain genera, in 



